1. Field of the Invention
The present invention relates to a method of producing hardened Al-Mg alloy forming sheets having a high strength and a superior corrosion resistance which are useful for easy open can ends or the like.
2. Description of the Prior Art
Conventionally, in making easy-open type can ends, there have been employed work hardened sheets fabricated from aluminum alloys including Mg as a primary alloying element, e.g., AA 5052, AA 5082, AA 5182 or the like in which cold rolling has been practiced to increase the strength of the resulting hardened sheets and further baking treatments of corrosion-resistant coatings applied to the sheets have been practiced. The hardened sheets in the as-cold-rolled state are susceptible to deterioration of the mechanical properties due to natural aging and are poor in formability. Therefore, stabilizing treatments at temperatures not exceeding 200.degree. C for a period of several hours have been conducted in certain cases. Further, when the hardened sheets are subjected to leveling operations for improving the flatness, nonuniform residual strain will be introduced in the resulting sheets. Therefore, when the coatings are applied onto such sheets without any treatment and baked, the internal residual strain causes warpage or distortion in the sheets which will cause problems in subsequent operations. Therefore, in certain cases, flattened hardened sheets have been heat-treated in a horizontally fixed state in order to remove irregular stress. The heat treatments have been usually carried out over a period of at least several hours at 150.degree. C. or less.
The stabilizing heat treatments as described above will precipitate compounds, such as .beta.-phase (Al.sub.8 Mg.sub.5), S-phase (Al.sub.2 CuMg), etc., which are electrochemically less noble relative to the matrix, along grain boundaries and working texture (disorder of crystal grains called "dislocation"), thereby resulting in a considerable reduction of the corrosion resistance of the alloys.